This application relates generally to combustors and, more particularly, to gas turbine combustors including pilot fuel injectors.
Air pollution concerns worldwide have led to stricter emissions standards both domestically and internationally. Aircraft are governed by both Environmental Protection Agency (EPA) and International Civil Aviation Organization (ICAO) standards. These standards regulate the emission of oxides of nitrogen (NOx), unburned hydrocarbons (HC), and carbon monoxide (CO) from aircraft in the vicinity of airports, where they contribute to urban photochemical smog problems. Most aircraft engines are able to meet current emission standards using combustor technologies and theories proven over the past 50 years of engine development. However, as aircraft fly in higher altitudes, such combustor technology may not be applicable. In particular, such technology is not effective for supersonic aircraft as they fly through the stratoshperic ozone.
To minimize potentially detrimental emissions caused during operation at higher altitudes, at least some known aircraft engines include multi-stage radial axial (MRA) combustors. MRA combustors include a plurality of fuel/air mixing tubes and an independent pilot fuel injector system. The pilot fuel injector system supplies fuel to the combustor during start-up and low power operations to facilitate low-power engine flame stability, and during increased power operations, fuel is staged independently through the fuel/air mixing tube while increasing a fuel/air ratio supplied to the pilot fuel injector system.
At least some known pilot fuel injector systems include multiple piece assemblies including a fuel stem and an integral mixer/heat shield. The fuel stem is inserted within a cavity defined within the mixer/heat shield and includes a plurality of structural extensions spaced circumferentially for securing an end of the fuel stem within the mixer/heat shield and prevents contact with the mixer/heat shield.
To thermally insulate the fuel stem from hot gases flowing within the combustor, the heat shield is placed around the fuel stem. The heat shield is thus exposed to higher temperatures than the fuel stem and is typically cast from high temperature alloys which possess high temperature and stress capabilities without sacrificing useful life requirements. Over time, because of continued exposure to high temperatures, the heat shield may need to be replaced. Because the heat shield and mixer comprise the same assembly, when the heat shield is replaced the mixer must also be removed. As a result, multiple spare parts are needed to maintain the pilot fuel injector system.
In an exemplary embodiment, a combustor for a gas turbine engine includes a pilot fuel injection assembly including a replaceable heat shield. The pilot fuel injection assembly also includes a fuel stem and a mixer divider. The fuel stem includes a first end, a second end, and a body extending therebetween. An alignment slot is defined within a portion of the interface between the fuel stem body and the second end. The mixer divider is attached to the fuel stem second end and defines a cavity between the fuel stem and the mixer divider. The heat shield is removably coupled to the mixer divider with a plurality of transfer tubes, and defines a cavity between the mixer divider and the heat shield.
During assembly of the pilot fuel injection assembly, as a portion of the fuel stem is received within the mixer divider, the fuel stem slot positions the mixer divider in alignment with respect to the fuel stem. The heat shield is then held in place radially around the mixer divider and the transfer tubes are extended from the heat shield to the mixer divider. Because the transfer tubes are attached only to the heat shield, a slip joint is created between the transfer tubes and the mixer divider. More specifically, because a first end of each transfer tube is attached to the mixer divider, and a second end of each transfer tube is inserted through the heat shield, a slip joint is created. As a result, during operation, vibrational loading induced within the heat shield is transmitted to the combustor through the fuel stem. Furthermore, the slip joint eliminates thermal growth differential between the heat shield and the mixer divider, and enables the heat shield to be replaced without removing the fuel stem from the mixer divider.